Hi Andrew,
First thing to do here is to really decide upon a design for the city. At the moment, it just looks like a bunch of pencils all stood upright , there is no sense of architecture, no signs of habitation, and nothing to gague scale from. If you look at all the best fictional cities in film, they all have a visual rhythm to them so the viewer can quickly see what type of buildings they are, their size, and how you would move through the buildings.

Once you have decided upon a design and what you want to show in the city, you need to find an interesting way to show it. That means experimenting with different perspectives to find something that converys the right feeling. Maybe you want to show how oppressive the city is and will go for a street level shot looking up at the surrounding buildings, or perhaps a view from above looking down at an angle to show how expansive it is and how it integrates with the surrounding environment...

Finally, once you have those things nailed down, find a colour palette that really works with that environment. Look at photos for light and shadow colour at different times of day and come up with something that has a very realistic sense of light, shadow, saturation and depth. Take a look at these 2 images which both do this well:

I don't have the names of the artists to hand, but neither of these images are mine. Hope it helps!